1. Field of the Invention
The present invention relates to an optical fiber and, more particularly, to a large-diameter optical fiber having the core diameter of not less than 100 .mu.m.
2. Related Background Art
There are a variety of causes of optical loss in an optical fiber. Typical causes of the optical loss in the optical fiber are bending, and modal dispersion in the case of large-diameter optical fibers having the core diameter of not less than 100 .mu.m. It is thus important to reduce these losses.
Incidentally, an example of the optical fiber is disclosed in Japanese Laid-open Patent Application No. 7-218734. This optical fiber has a core made of silica or optical glass, and a cladding made of a cured resin composition.
Another example of the optical fiber is disclosed in Japanese Laid-open Patent Application No. 3-245108. As shown in FIG. 7, this optical fiber 100 has a core 101, which contains a glass base material and in which a refractive index profile of a parabolic profile type is formed across the whole core diameter, and a cladding 102, which is made of a plastic material having a constant index of refraction smaller than those of the core 101. The refractive index profile of the core 101 is established by a concentration distribution of a dopant, such as Ge, added into the core 101. The refractive index of the cladding 102 is not less than 0.97 and not more than 0.985 times that of the outer peripheral portion of the core 101, and a difference is made in the refractive index at the border between the core 101 and the cladding 102, thereby attaining a high numerical aperture (NA).
Further, Japanese Patent Publication No. Hei 3-18161 discloses the optical fiber having a center core, a low index core disposed outside thereof and having a smaller refractive index than that of the center core, and a cladding region disposed further outside thereof. This optical fiber has zero dispersion in the 1.5 .mu.m band and is thus a single-mode optical fiber therein. Further, the relative index difference thereof is more than 0.004 but not more than 0.014.